A spectrograph is a device for recording the spectral composition of light emerging from an entrance aperture, such as a slit. A simple, prior-art spectrograph configuration uses a conventional Czerny-Turner monochromator consisting of a planar reflection grating and a pair of concave spherical mirrors. Radiation emerging from the entrance slit is collimated by the first mirror, and the collimated light is chromatically dispersed by the grating to separate the color content along different angles. The second mirror then focuses these angularly dispersed colors into spatially separated images, and an exit slit passes only a small color range for measurement by a single detector channel. Data may be gathered over an extended spectral range by rotating the grating, which causes different color regions of the spectrally dispersed image to coincide with the exit slit.
Spectrograph configurations based upon the Czerny-Turner monochromator are slow due to the serial nature of the spectral data acquisition. Speed limitations are further pronounced in applications such as Raman spectroscopy, where weak signal levels require long integration times for each spectral data point on a photon-counting detector.
Recent advances have resulted in improvements to both spectrum acquisition time and the signal-to-noise ratio in spectrographic instruments. For example, spectral data may be acquired in parallel on a two-dimensional detector, which replaces the exit slit, thus allowing the grating to remain fixed while the detector array is illuminated by an extended spectral range from the monochromator. The grating may then be stepped to another position or replaced with a different grating to acquire data in a different spectral range.
If a simple entrance slit is being imaged onto a CCD detector, the detector signals may be added or binned in the constant color direction directly on the detector device, thus enhancing sensitivity and signal-to-noise ratio. If increased spatial resolution is also required; for instance, if the entrance slit is replaced by an array of optical fibers from different light sources, then a modified imaging Czerny-Turner monochromator may be constructed in conjunction with a 2-D image sensor by replacing the spherical mirrors with toroidal mirrors to correct for astigmatism. Another recent development in spectrograph technology is the use of volume holographic transmission gratings in place of the more conventional surface relief reflection gratings of both the ruled and holographic type. Such volume gratings can be used with on-axis transmission lenses as imaging elements in place of the reflective mirrors, resulting in an efficient system and a very compact package as evident by U.S. Pat. No. 5,011,284, assigned to the assignee of the present invention. Volume gratings are also capable of very high dispersion, enhancing the spectral resolution for a given focal length and image size.
While the use of a detector array in place of an exit slit enhances spectrographic performance and reliability, the use of a rotatable grating, grating turret, or any moving component remains a serious drawback. Thus, even with recent advances in the prior art, there remains an unsatisfied need for a compact and efficient spectrographic instrument wherein a two-dimensional detector array may be employed for both high spectral resolution and a large spectral bandwidth without the need for moving parts.